Ion specificity of the zeta potential of α-alumina, and of the adsorption of p-hydroxybenzoate at the α-alumina-water interface

Abstract

The influence of inorg. anions (NO-3, I-, Br-, Cl-, SO42-, and S2O32-) and divalent cations (Ca2+ and Mg2+) on the zeta potential and isoelec. point of α-alumina in aq. medium has been studied. The effect of the anions is highly ion specific even at salt concns. as low as 5 × 10-4 M. This unexpected finding is in line with a recent report. It is also in agreement with an earlier theor. prediction. The results are consistent with the classical Hofmeister series, except for the case of NO3-. Divalent anions (SO42- and S2O32-) decrease the magnitude of the zeta potential of α-alumina in aq. medium, more precisely; S2O32- produced large neg. zeta potential (∼-12 to -47 mV) within the pH range of the study without the isoelec. point (IEP) of α-alumina. However, the SO42- decreased the zeta potential of α-alumina of different magnitudes (max. ∼25 mV at both ends of the exptl. acidic and basic pH scale) with a minor shift of the IEP (∼0.5 unit) toward lower pH. Ca2+ and Mg2+ produce zeta potentials of α-alumina roughly equal to that of neat α-alumina but slightly higher than that of Na+ at both sides of the IEP. We have shown further that the same ion specificity or equivalently competitive ion effects occur with the adsorption d. of p-hydroxybenzoate onto α-alumina surfaces. The sequence of anions (with common cation) for the adsorption d. of p-hydroxybenzoate on the α-alumina surfaces follows the Hofmeister series sequence: S2O32- < SO42- < Cl- > Br- > I- > NO3-. The divalent cations (Ca2+ and Mg2+) exhibit a roughly equiv. effect on the adsorption of p-hydroxybenzoate onto α-alumina surfaces. Using the frequency shifts of ν as(-COO-) and ν s(-COO-) in the DRIFT spectra of p-hydroxybenzoate after adsorption and other characteristic peaks, we have demonstrated that p-hydroxybenzoate forms outer-sphere complexes onto α-alumina surfaces at pH 5 and 6 and inner-sphere complexes at pH 7, 8, and 9 in the presence of 5 × 10-4 M NaCl(aq).